They were accused of not having correct gun permits but their paperwork - issued by the British government - was in order.

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Prince Charles has also written to the family of one of the men who he met when he was awarding Afghanistan medalsCredit: Getty Images

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We detailed the soldiers' plight last month

Yet they were still thrown in a hell-hole jail alongside terrorists, murderers and rapists.

Four of the men are ex-Paras and Charles, who is Colonel in Chief of The Parachute Regiment, sent a moving letter to the parents of one, John Armstrong, 28.

John and Helen Armstrong, of Wigton, Cumbria, had written to the future King in the hope that he could help the campaign to free the men.

His assistant private secretary explained he was unable to intervene personally but said: "The Prince of Wales was deeply saddened to hear of the distressing situation your son John, and indeed your family, finds itself in.

"The prolonged separation, concern for health and wellbeing and financial strain as you await the outcome of the judicial process must be a very great burden on you all.

"His Royal Highness is grateful to you for taking the trouble to write to him as you did and he feels so deeply for you. I send you every good wish for a speedy conclusion to this matter so that John may return to his loved ones soon." Charles met John in 2011 while presenting medals to members of 3 Para who had recently returned from a six-month tour of Afghanistan.

Another campaigner who has lobbied the British government to help secure the men's release wrote to Prince Harry, who served for ten years in the military and helped organise the Invictus Games in London for wounded vets.

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Billy Irving has only seen his baby son for the duration of a few weeksCredit: Chris Eades

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John Armstrong has lost more than three stone in jail

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Paul Towers was thrown into jail for a crime he didn’t commit

His office replied: "Prince Harry has asked me to send you his very best wishes and hopes that there will be a resolution soon." The six were delighted to hear of the support from the Royals.

John said from his prison cell in Chennai: "I was proud to receive my medals from Prince Charles and I'm touched to know he is thinking of us now in our current plight.

"We all served the Queen loyally so we are delighted to know the Royal family is supporting us in our hour of need.

"It also gives us a real boost to know Prince Harry hopes we are home soon.

"He obviously served himself in the very recent past and is a huge supporter of veterans."

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She said: "The men have very little communication so any snippets of news are much appreciated.

"John was definitely buoyed by the news of the support they have from the Royal family. It's support such as this that keeps them going." Alongside John in prison are Nick Dunn, 30, of Ashington, Northumberland; Ray Tindall, 41, of Chester; Nicholas Simpson, 45, of Catterick, North Yorkshire; Paul Towers, 52, of Pocklington, East Yorkshire; and Billy Irving, 35, of Oban, Scotland.

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Nicholas Simpson served in the army for 24 years before his wrongful imprisonment

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Divorced dad-of-three Ray Tindall, an elite sniper, did 12 years with the 1st Battalion

Nick Dunn's sister Lisa, 37, said: "It's a tremendous boost to us knowing that Prince Harry has shown his support by hoping this dreadful injustice be resolved quickly.

"I understand he can't interfere in India's legal processes but he fully supports all veterans and will appreciate these men need to be released from prison.

"They need to be back with their loved ones." The men were on an anti-piracy vessel protecting ships from Somali pirates when they were detained in October 2013 as they stopped at an Indian port for emergency fuel.

The coastguard accused them of illegally importing unlicensed weapons even though they had permits to carry the guns and ammunition.

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The PM failed to raise the mens' plight when she had talks with India PM Narendra Modi in ChinaCredit: Getty Images

The men pleaded they were just doing their job but a local judge ruled they were a threat to national security and sentenced them to five years.

Lawyers are preparing an appeal which will be heard next month. A bail hearing held last week was adjourned and will also now be heard next month.

A Foreign Office spokesman said: “We recognise what a difficult time this is for those involved.

“Our ministers have taken significant action in this case, raising it at the highest levels in the Indian government, and will continue to do so.

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International Trade Secretary Liam Fox and Development Secretary Priti Patel also didn’t bring up their case on their visits to Delhi

"Earlier this year the then Foreign Office minister Hugo Swire travelled to India to meet the men personally, as well as meeting their families in Carlisle.

"Alok Sharma, the new Minister for Asia, also raised it in India in July in his first official visit, days after being appointed.

"Our staff in India and the UK have also been in regular contact with all six men since their arrest and are continuing to support them and their families, working to make sure their welfare is protected in prison.

“We cannot interfere with India’s independent legal system, just as other countries cannot interfere with ours, but we will continue efforts to make sure this case is resolved swiftly.”